Figure 7 of Mitochondrial DNA Analysis at the FBI Laboratory by Isenberg and Moore (Forensic Science Communications, July 1999)

U.S. Department of Justice

Federal Bureau of Investigation

July 1999 Volume 1 Number
2

Mitochondrial DNA Analysis
at the FBI Laboratory

Figure 7
Cycle Sequencing

The cycle sequencing process
is very similar to the polymerase chain reaction, but the reaction
is terminated by the incorporation of dideoxynucleotide triphosphates
(ddNTPs) before the DNA strands can extend to their full length.
Fluorescently labeled ddNTPs are mixed with regular dNTPs, a
primer, and the DNA template for the reaction. During the course
of the reaction, the ddNTPs compete with the dNTPs for positions
on the DNA template, resulting in a pooled set of fragments differing
by a single base in length. At the end of each strand is a fluorescent
dye molecule, the color of which depends on the base at the terminal
position of the strand. The different dye colors identify the
base present at the end of each strand, thus providing a method
for determining the sequence of the entire strand.